Multi-layer polymeric film for packaging ovenable meals

ABSTRACT

A heat-sealable peelable multi-layer laminated polymeric film comprising a polymeric substrate layer having on one side thereof a polymeric heat-sealable peelable layer and having on the opposite side thereof a polymeric shrinkable layer, wherein said shrinkable layer has a degree of shrinkage in a first dimension of about 10-80% over the temperature range 55 to 100° C., a ratio of shrinkage at 100° C. said first dimension relative to a second, orthogonal dimension in the range of 1:1 to 10:1, and further comprising one or two intermediate layer(s) wherein an intermediate layer is disposed between the substrate layer and the shrinkable layer and/or between the substrate layer and the heat-sealable peelable layer, and one or more layer(s) of an electrically conductive material; and use thereof as packaging for ovenable meals or as a lid on an ovenable container wherein the packaging is self-peeling and self-venturing during a cooking cycle.

This invention relates to a multi-layer polymeric film, and inparticular to a laminated multi-layer polyester film. The film issuitable for use as packaging for ready-prepared ovenable meals,including use as the sole packaging means for the ovenable meal as wellas use as a lid for a container for the ovenable meal particularly for acontainer which does not already contain a metallic component.

Plastic containers have been increasingly used in packagingapplications, such as food packaging, and in particular for packagingconvenience foods, for example ready-prepared ovenable meals which arewarmed either in a microwave oven or in a conventional oven. Often thecontainer comprises a polymeric material onto which has been deposited athin metal layer, such as metallised (particularly flash-metallised) PETcartonboard. For example, the container may be produced from PET whichhas been metallised to an optical density in the range of about 0.01 to4.0 and which is laminated to cartonboard. Such containers have beenreferred to as “susceptor” containers and are disclosed in, forinstance, GB-A-2280342, EP-A-0563442, GB-A-2250408 and GB-A-2046060. Thepurpose of the susceptor container is to increase the heating effect,typically at predetermined locations, for instance, at a surface of thefoodstuff to give it the appearance of having been “browned” or“crisped” and having been cooked by conventional, non-microwave cookingmethods.

Such susceptor containers comprise a susceptor material which is amaterial which heats up rapidly under the influence of microwave energyand typically comprises layers of conductor particles (usually metallicparticles). If the foodstuff is located in contact with, or adjacent to,the susceptor material, it receives heat by conduction to its surfacewhich gives rise to a browning or crisping effect, whilst the remainderof the foodstuff is being heated throughout by a conventional microwaveeffect. Thus, the susceptor material may lie between the product and asubstrate forming part of the container which holds the product, or itmay lie to the outside of the substrate provided the susceptor effect iscapable of being applied through the substrate to the product.

Ovenable containers for ready prepared meals often require lids whichcan both seal the container, in order to prevent leakage and drying outof the packaged contents during storage, and which can also be easilypeeled from the container on opening. For those applications requiringlids, it has generally been necessary to provide a susceptor material inor on the portion of the container to which the lid has been attachedvia a heat-seal bond. This portion of the container is generally knownas the “seal area”. The rapid temperature increase of the susceptormaterial during the cooking cycle causes the heat-seal bond to softenand therefore allows easier peeling of the lid once cooking iscompleted.

Other important requirements of the lids are that they should not stickto the packaged contents and that they should be able to withstand theheat generated in the oven.

Container lids normally comprise a film comprising a flexible substrateand a sealable coating layer, and are often referred to as “lidding”films. Oriented polymeric film, particularly biaxially orientedpolyester film, has previously been used as the flexible substrate forlidding films. In prior art lidding films, the heat-sealable layer isoften applied to the substrate in an “off-line” coating step, i.e. afterany stretching and subsequent heat-setting operation employed during themanufacture of the substrate. The heat-sealable layers of such films aregenerally applied using organic solvent, which is not suitable for“in-line” coating, and can cause sticking or blocking of the film duringthe normal winding operations used during film manufacture. In addition,the organic solvents used may be harmful, hazardous in use, or toxic anddetrimental to the environment.

WO-A-96/19333 discloses a lidding film produced by an in-line coatingprocess which avoids the use of substantial amounts of organic solvents.The film comprises a substrate and a coating layer of a copolyester of(a) 40 to 90 mole % of at least one aromatic dicarboxylic acid, (b) 10to 60 mole % of at least one aliphatic dicarboxylic acid, (c) 0.1 to 10mole % of at least one dicarboxylic acid comprising a free acid groupand/or a salt thereof, (d) 40 to 90 mole % of at least one glycol havingfrom 2 to 12 carbon atoms, and (e) 10 to 60 mole % of at least onepolyalkylene glycol. The coating layer is applied as an aqueousdispersion or solution either before the film substrate is stretched orbetween the stretching steps of a biaxial stretching process. Theprocess disclosed in WO-A-96/19333 is limited to certain types ofcoating compositions, i.e. those that are soluble or adequatelydispersible in water.

The manufacture of sealed containers using lidding films involves theformation of a seal between the lidding film and the container. Thisseal is formed by placing the lid on top of the container and applyingheat and pressure in order to soften or melt the sealable coating layerso that it adheres to the surface of the container and forms aneffective seal between the lid and the container. Unfortunately, a sealwhich is strong enough to prevent leakage of the contents often resultsin difficulties in removing the lid when the container is to be opened.In particular, the lid may tear during peeling and portions of the filmlid may fall into the contents of the container thereby spoiling thefood. This problem is a particular disadvantage for containers which donot contain a susceptor material since there may have been insufficientheat transfer to the heat-seal bond. A strong seal and easy peelingproperties, i.e. a clean peel, are desirable at both low, e.g. ambient,and high temperatures, e.g. after heating the packaged food contents inan oven. It would be desirable to eliminate the difficulties encounteredduring peeling. It would also be desirable to provide a lid which willfunction satisfactorily on all types of containers, rather than justsusceptor containers.

A further consideration with ready-prepared convenience meals is thatwater vapour is driven from the food during the cooking cycle. If thesteam thereby produced is not vented, the build-up of pressure may causethe packaging, for instance the film lid, to burst, causing fragments ofthe packaging to contaminate the contents of the container. Previouspackaging for ovenable ready-prepared food containers generally requiredthat the user pierce the packaging to prevent this. However, the needfor piercing prior to warming the food in its container is oftenforgotten or not understood by the user. It would be advantageous toprovide packaging which did not require the user to pierce it beforecooking.

It is an object of this invention to provide a film suitable for use inpackaging ready-prepared ovenable meals, which exhibits superior openingor peeling characteristics and is self-venting. It is a further objectof this invention to provide a film suitable for use in packagingready-prepared ovenable meals, which exhibits superior opening orpeeling characteristics and is self venting, and which is particularlysuitable for use as a lid on containers which do not contain a susceptormaterial as well as on containers which do contain a susceptor material.It is a particular object of the invention to provide a film suitablefor use in packaging ovenable meals which is self-peeling during thecooking cycle.

According to the present invention, there is provided a heat-sealablepeelable multi-layer laminated polymeric film comprising a polymericsubstrate layer having on one side thereof a polymeric heat-sealablepeelable layer and having on the opposite side thereof a polymericshrinkable layer, wherein said shrinkable layer has a degree ofshrinkage in a first dimension of about 10-80% over the temperaturerange 55 to 100° C., and a ratio of shrinkage at 100° C. said firstdimension relative to a second, orthogonal dimension in the range of 1:1to 10:1, and further comprising intermediate layer(s) disposed betweenthe substrate layer and the shrinkable layer and/or between thesubstrate layer and the heat-sealable peelable layer, and one or morelayer(s) of an electrically conductive material.

In one embodiment, a second intermediate layer is present such thatthere is a first intermediate layer disposed between the substrate layerand the shrinkable layer and a second intermediate layer disposedbetween the substrate and the heat-sealable peelable layer. Preferably,however, only one intermediate layer is present. Preferably theintermediate layer is disposed between the substrate layer and theshrinkable layer.

The layers of electrically conductive material preferably comprise anelemental metal. However, any other suitable electrically conductivematerial approved for use with food packaging may be used, for instancegraphite particles. Of course, where the film is intended for use aspackaging for a microwaveable ready-prepared meal, the conductivematerial must be capable of absorbing the microwave energy. For brevityand convenience, the invention is hereinafter described with referenceto the preferred embodiment wherein the electrically conductive layer isa metallic layer. It will nevertheless be appreciated that the followingis applicable to other types of electrically conductive layers.

The metallic layer(s) is/are disposed between one or more of thefollowing pairs of layers: (i) the substrate layer and the intermediatelayer; (ii) the shrinkable layer and the intermediate layer; (iii) thesubstrate layer and the heat-sealable layer; (iv) the intermediate layerand the heat-sealable peelable layer; and (v) the substrate layer andthe shrinkable layer. Preferably, the metallic layer(s) is/are between(i) the substrate layer and the intermediate layer and/or (ii) theshrinkable layer and the intermediate layer. In one embodiment, there isonly one metallic layer, and this is preferably between the substratelayer and the intermediate layer.

The film of the present invention is suitable for use as packaging inready-prepared ovenable meals and is self-opening under the action ofthe heat experienced during the cooking cycle. The presence of themetallic layer increases the efficiency of heat transfer within thefilm, thereby providing superior and more rapid self-peeling orself-opening characteristics during the cooking cycle in relation toexisting films.

The film of the present invention is capable of providing a strong seal,thereby reducing the risk of content leakage during transport andstorage, while at the same time providing a seal which is readily openedduring the cooking cycle. Previously, it was generally considered thatthere had to be a trade-off between these two apparently conflictingcharacteristics.

The film of the present invention is suitable for use in two primaryapplications. Firstly, the film may be used as a lidding film forovenable containers, particularly ovenable containers which do notthemselves contain a metallic component. Thus, it is particularlysuitable for use on containers and trays which do not contain susceptormaterial, either in the food-contact portion of the container or, moreparticularly, in the seal area of the container. The metallic componentof the multilayer film replaces the need for the container or tray tocomprise a metallic component, particularly in the seal area. Of course,even though the lidding film of the present invention avoids the needfor susceptor material in the seal area of the container, it may stillbe desirable for the container to comprise susceptor material in otherareas in order to “brown” or “crisp” the food substance, as noted above.Secondly, the film of the present invention may be used as the solepackaging means for ready-prepared ovenable meals in the absence of acontainer, such packaging being generally known has “flow-wrap”packaging, particularly the horizontal or vertical “form-fill and seal”type packaging well-known in the art.

Thus, in a first embodiment, the film of the present invention isintended for use by heat-sealing the film onto an ovenable container.During the cooking cycle the food product contained in the sealedcontainer heats up and steam is produced. Heat from the steam and fromgeneral radiant heat is transferred to the film lid. Once thetemperature of the shrinkable layer reaches its shrink initiationtemperature, it starts to shrink, thereby applying a shear force to thefilm lid. In addition, the strength of the heat-seal bond between thecontainer and the film lid will start to reduce as the temperatureincreases, particularly when the temperature passes the glass transitiontemperature of the heat-seal layer polymer. The presence of the metalliclayer increases the rate of heat transfer to the shrinkable layer and tothe heat-sealable layer. As the bond between the film lid and the trayweakens, the shear force exerted by the shrinkable layer causes the lidto curl back on itself and peel open during the cooking cycle. Theself-peeling lid therefore allows the venting of steam generated duringthe cooking cycle. This is particularly advantageous for solid foods,particularly dough-based foods such as pizzas, because it prevents thebase becoming soggy or water-logged. The self-peeling lid also avoidsany difficulties, such as tearing of the film lid, which are associatedwith opening of the container by the user once the cooking cycle iscomplete.

In a second embodiment, the film of the present invention is intendedfor use as the sole packaging means for an ovenable meal. The film isheat-sealed to itself in order to completely envelop the food product.As before, once the temperature of the shrinkable layer reaches itsshrink initiation temperature, it starts to shrink, thereby applying ashear force to the film as a whole. In addition, the strength of theheat-seal bond will start to decrease as the temperature increases. Theshear force exerted by the shrinkable layer causes the film to separateat the heat-seal bond, curling back on itself and exposing the foodproduct, with the advantages noted above. Such packaging is particularlyuseful for products such as pizzas, French bread, garlic bread and thelike.

In both embodiments, the shear force producible by the shrinkage of theshrinkable layer is greater than the strength of the heat-seal bondprovided by the heat-sealable layer.

A multi-layer laminated film according to the present inventiontypically exhibits a heat-seal value (at ambient temperatures) in therange of 200 to 1400 g/25 mm², preferably in the range of about 200 toabout 1000 g/25 mm², when sealed to a standard APET/CPET tray (acomposite material having an amorphous polyethylene terephthalate layeron top of a crystalline polyethylene terephthalate layer). Typical heatseal strengths of the film to itself are in the range of 600-800 g/25mm².

The thickness of the multi-layer laminated film is typically from about20 to about 350 μm, preferably from about 20 to about 250 μm, preferablyfrom about 40 to about and 150 μm, and more preferably from about 60 toabout 100 μm.

The primary characteristic of the film of the present invention is thatit exhibits self-opening during the cooking cycle. In order for this tooccur, the shrinkable layer must have a degree of shrinkage in one orboth of said first and second dimension(s) that is greater than thedegree of shrinkage of the substrate layer in that dimension. Further,the shear force produced by the shrinkage of the shrinkable layer shouldbe sufficient to break the heat-seal bond, i.e. this shear force shouldbe greater than the heat-seal bond strength, and thereby cause the filmto self-peel during the cooking cycle.

As the skilled person is aware, the shrinkage characteristics of a filmare determined by the stretch ratios and degree of heat-setting employedduring its manufacture. The shrinkage behaviour of a film which has notbeen heat-set corresponds to the degree to which the film has beenstretched during its manufacture. In the absence of heat-setting, a filmwhich has been stretched to a high degree will exhibit a high degree ofshrinkage when subsequently exposed to heat; a film which has only beenstretched by a small amount will only exhibit a small amount ofshrinkage. Heat-setting has the effect of providing dimensionalstability to a stretched film, and “locking” the film in its stretchedstate. Thus, the shrinkage behaviour of a film under the action of heatdepends on whether, and to what extent, the film was heat-set after thestretching operation(s) effected during its manufacture. In general, afilm which has experienced a temperature T₁ during the heat-settingoperation will exhibit substantially no shrinkage below temperature T₁when subsequently exposed to heat after manufacture.

Accordingly, in order that the shrinkable layer exhibit the requiredshrinkage characteristics in the final laminated film, it is preferredthat the shrinkable layer is not heat-set after stretching has beeneffected. However, the skilled person will recognise that filmsaccording to the present invention may also be produced using shrinkablelayers which have been partially heat-set.

In a preferred embodiment, the substrate layer is a dimensionally stablefilm which exhibits substantially no shrinkage under the action of heat,and the shrinkable layer is a layer which shrinks in one dimension underthe action of heat. The film of this embodiment, under the action ofheat, therefore peels back on itself in one direction only. In thispreferred embodiment, the substrate layer may be a biaxially-orientedheat-set polymeric layer and the shrinkable layer may be auniaxially-oriented layer which has not been heat-set.

As the skilled person is aware, the dimensions of a polymeric film aredefined in terms of the “machine direction” and the “transversedirection”, which correspond to the axes of the film productionapparatus. The machine direction is the direction of travel along thefilm production line and corresponds to the lengthways dimension of thefilm. The transverse direction is the direction orthogonal to thedirection of travel of the film during manufacture and corresponds tothe widthways dimension of the film. Conveniently, the shrinkable layerof the preferred embodiment described above exhibits uniaxial shrinkagein the transverse direction.

The present invention is not limited, however, to films which shrink inonly one dimension. For instance, the shrinkable layer may shrink in afirst dimension as well as in a second, orthogonal dimension. The degreeof shrinkage in one dimension may be the same as, or different to, thedegree of shrinkage in the second, orthogonal direction. Thus, in analternative embodiment, the substrate layer is a dimensionally stablefilm which exhibits substantially no shrinkage under the action of heat,and the shrinkable layer is a layer which shrinks in two dimensionsunder the action of heat. The film of this embodiment, under the actionof heat, therefore peels back on itself in two directions. In thispreferred embodiment, the substrate layer may be a biaxially-orientedheat-set polymeric layer and the shrinkable layer may be abiaxially-oriented layer which has not been heat-set.

The present invention is also not limited to films in which thesubstrate exhibits substantially no shrinkage. For instance, thesubstrate may also exhibit shrinkage in one or two dimensions providedthat the degree of shrinkage of the substrate layer in a given dimensionis less than the degree of shrinkage of the shrinkable layer in thatdimension. Thus, in an alternative embodiment the substrate layer andthe shrinkable layer may be layers which have different compositionsand/or have been produced using different manufacturing conditions (suchas different stretch ratios and/or heat-set processing conditions) andwhich therefore have different relative degrees of shrinkage in a givendimension.

The respective layers of the multi-layer laminated film are described inmore detail below.

The substrate is a self-supporting film or sheet by which is meant afilm or sheet capable of independent existence in the absence of asupporting base. The substrate may be formed from any suitablefilm-forming material. Thermoplastic polymeric materials are preferred.Such materials include a homopolymer or copolymer of a 1-olefin, such asethylene, propylene and but-1-ene, a polyamide, a polycarbonate, PVC,PVA, polyacrylates, celluloses and particularly a synthetic linearpolyester.

The synthetic linear polyesters useful as the substrate may be obtainedby condensing one or more dicarboxylic acids or their lower alkyl (up to6 carbon atoms) diesters, eg terephthalic acid, isophthalic acid,phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinicacid, sebacic acid, adipic acid, azelaic acid, 4,4′-diphenyldicarboxylicacid, hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly an aliphatic or cycloaliphatic glycol,e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycoland 1,4-cyclohexanedimethanol. An aromatic dicarboxylic acid ispreferred. An aliphatic glycol is preferred.

In a preferred embodiment, the polyester is selected from polyethyleneterephthalate and polyethylene naphthalate. Polyethylene terephthalate(PET) is particularly preferred.

The substrate may also comprise a polyarylether or thio analoguethereof, particularly a polyaryletherketone, polyarylethersulphone,polyaryletheretherketone, polyaaryletherethersulphone, or a copolymer orthioanalogue thereof. Examples of these polymers are disclosed inEP-A-001879, EP-A-0184458 and U.S. Pat. No. 4,008,203. Blends of suchpolymers may also be employed. Other thermoset resin polymeric materialssuitable for use as the substrate include addition-polymerisationresins, such as acrylics, vinyls, bis-maleimides and unsaturatedpolyesters; formaldehyde condensate resins, such as condensates withurea, melamine or phenols, cyanate resins, functionalised polyesters,polyamides or polyimides.

The substrate may comprise one or more discrete layers of the abovefilm-forming materials. The polymeric materials of the respective layersmay be the same or different. For instance, the substrate may compriseone, two, three, four or five or more layers and typical multi-layerstructures may be of the AB, ABA, ABC, ABAB, ABABA or ABCBA type.Preferably, the substrate comprises only one layer.

Formation of the substrate may be effected by conventional techniqueswell-known in the art. Conveniently, formation of the substrate iseffected by extrusion, in accordance with the procedure described below.In general terms the process comprises the steps of extruding a layer ofmolten polymer, quenching the extrudate and orienting the quenchedextrudate in at least one direction.

The substrate may be uniaxially-oriented, but is preferablybiaxially-oriented, as noted above. Orientation may be effected by anyprocess known in the art for producing an oriented film, for example atubular or flat film process. Biaxial orientation is effected by drawingin two mutually perpendicular directions in the plane of the film toachieve a satisfactory combination of mechanical and physicalproperties.

In a tubular process, simultaneous biaxial orientation may be effectedby extruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation.

In the preferred flat film process, the substrate-forming polyester isextruded through a slot die and rapidly quenched upon a chilled castingdrum to ensure that the polyester is quenched to the amorphous state.Orientation is then effected by stretching the quenched extrudate in atleast one direction at a temperature above the glass transitiontemperature of the polyester. Sequential orientation may be effected bystretching a flat, quenched extrudate firstly in one direction, usuallythe longitudinal direction, i.e. the forward direction through the filmstretching machine, and then in the transverse direction. Forwardstretching of the extrudate is conveniently effected over a set ofrotating rolls or between two pairs of nip rolls, transverse stretchingthen being effected in a stenter apparatus. Alternatively, the cast filmmay be stretched simultaneously in both the forward and transversedirections in a biaxial stenter. Stretching is effected to an extentdetermined by the nature of the polyester, for example polyethyleneterephthalate is usually stretched so that the dimension of the orientedfilm is from 2 to 5, more preferably 2.5 to 4.5 times its originaldimension in the or each direction of stretching. Typically, stretchingis effected at temperatures in the range of 70 to 125° C. Greater drawratios (for example, up to about 8 times) may be used if orientation inonly one direction is required. It is not necessary to stretch equallyin the machine and transverse directions although this is preferred ifbalanced properties are desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional restraint at a temperature above theglass transition temperature of the polyester but below the meltingtemperature thereof, to induce crystallisation of the polyester. Theactual heat-set temperature and time will vary depending on thecomposition of the film but should not be selected so as tosubstantially degrade the tear resistant properties of the film. Withinthese constraints, a heat set temperature of about 135° to 250° C. isgenerally desirable, as described in GB-A-838708.

Where the substrate comprises more than one layer, preparation of thesubstrate is conveniently effected by coextrusion, either bysimultaneous coextrusion of the respective film-forming layers throughindependent orifices of a multi-orifice die, and thereafter uniting thestill molten layers, or, preferably, by single-channel coextrusion inwhich molten streams of the respective polymers are first united withina channel leading to a die manifold, and thereafter extruded togetherfrom the die orifice under conditions of streamline flow withoutintermixing thereby to produce a multi-layer polymeric film, which maybe oriented and heat-set as hereinbefore described. Formation of amulti-layer substrate may also be effected by conventional laminationtechniques, for example by laminating together a preformed first layerand a preformed second layer, or by casting, for example, the firstlayer onto a preformed second layer.

The substrate layer is suitably of a thickness from about 5 to about 200μm, preferably from about 10 to about 150 μm and particularly from about15 to about 40 μm.

The heat-sealable layer is any layer capable of forming a heat-seal bondto the surfaces of a container or to itself. Examples include polymericmaterials such as polyester, EVA or modified polyethylene. The polymermaterial of the heat-sealable layer should soften to a sufficient extentthat its viscosity becomes low enough to allow adequate wetting for itto adhere to the surface to which it is being bonded. The heat-seal bondis effected by heating to soften the polymer material of theheat-sealable layer, and optionally applying pressure, without melting(or otherwise affecting the structure or compromising the structuralintegrity of) the substrate layer and/or the shrinkable layer to anysignificant degree. Thus, the polymer of the heat-sealable layer shouldbegin to soften at a temperature such that the heat-seal bond can beformed at a temperature which is less than the melting temperature ofthe substrate layer or the shrinkable layer. In one embodiment, thepolymer of the heat-sealable layer should begin to soften at atemperature such that the heat-seal bond can be formed at a temperaturewhich is between about 5 and 50° C. below, preferably between about 5and 30° C. below, and preferably at least about 10° C. below the meltingtemperature of the polymer material of the substrate layer or theshrinkable layer.

The heat-sealable layer suitably comprises a polyester resin,particularly a copolyester resin derived from one or more dicarboxylicacid(s) or their lower alkyl (up to 14 carbon atoms) diesters with oneor more glycol(s), particularly an aliphatic or cycloaliphatic glycol,preferably an aliphatic glycol, and more preferably an alkylene glycol.Suitable dicarboxylic acids include aromatic dicarboxylic acids such asterephthalic acid, isophthalic acid, phthalic acid, or 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, and aliphatic dicarboxylic acids suchas succinic acid, sebacic acid, adipic acid, azelaic acid, suberic acidor pimelic acid. Suitable glycol(s) include aliphatic diols such asethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, 1,3-butane diol, 1,4-butane diol, 1,5-pentane diol,2,2-dimethyl-1,3-propane diol, neopentyl glycol and 1,6-hexane diol, andcycloaliphatic diols such as 1,4-cyclohexanedimethanol and1,4-cyclohexane diol. Ethylene glycol or 1,4-butanediol is preferred.

Preferably, the heat-sealable layer comprises a copolyester derived fromat least two dicarboxylic acids. Formation of the copolyester isconveniently effected in known manner by condensation, orester-interchange, at temperatures generally up to 275° C.

In a preferred embodiment, the copolyester of the heat-sealable layercomprises an aromatic dicarboxylic acid and an aliphatic dicarboxylicacid. A preferred aromatic dicarboxylic acid is terephthalic acid.Preferred aliphatic dicarboxylic acids are selected from sebacic acid,adipic acid and azelaic acid. The concentration of the aromaticdicarboxylic acid present in the copolyester is preferably in the rangefrom 45 to 80, more preferably 50 to 70, and particularly 55 to 65 mole% based on the dicarboxylic acid components of the copolyester. Theconcentration of the aliphatic dicarboxylic acid present in thecopolyester is preferably in the range from 20 to 55, more preferably 30to 50, and particularly 35 to 45 mole % based on the dicarboxylic acidcomponents of the copolyester. Particularly preferred examples of suchcopolyesters are (i) copolyesters of azeleic acid and terephthalic acidwith an aliphatic glycol, preferably ethylene glycol; (ii) copolyestersof adipic acid and terephthalic acid with an aliphatic glycol,preferably ethylene glycol; and (iii) copolyesters of sebacic acid andterephthalic acid with an aliphatic glycol, preferably butylene glycol.Preferred polymers include a copolyester of sebacic acid/terephthalicacic/butylene glycol (preferably having the components in the relativemolar ratios of 45-55/55-45/100, more preferably 50/50/100) having aglass transition point (T_(g)) of −30° C. and a melting point (T_(m)) of117° C.), and a copolyester of azeleic acid/terephthalic acid/ethyleneglycol (preferably having the components in the relative molar ratios of40-50/60-50/100, more preferably 45/55/100) having a T_(g) of −15° C.and a T_(m) of 150° C.

In an alternative embodiment, the heat-sealable layer comprises acopolyester derived from an aliphatic diol and a cycloaliphatic diolwith one or more, preferably one, dicarboxylic acid(s), preferably anaromatic dicarboxylic acid. Typical polyesters which providesatisfactory heat-sealable properties include copolyesters ofterephthalic acid with an aliphatic diol and a cycloaliphatic diol,especially ethylene glycol and 1,4-cyclohexanedimethanol. The preferredmolar ratios of the cycloaliphatic diol to the aliphatic diol are in therange from 10:90 to 60:40, preferably in the range from 20:80 to 40:60,and more preferably from 30:70 to 35:65. In a preferred embodiment thecopolyester is a copolyester of terephthalic acid with about 33 mole %1,4-cyclohexane dimethanol and about 67 mole % ethylene glycol. Anexample of such a polymer is PETG™T6763 (Eastman) which comprises acopolyester of terephthalic acid, about 33% 1,4-cyclohexane dimethanoland about 67% ethylene glycol and which is always amorphous. In analternative embodiment of the invention, the heat-sealable layer maycomprise butane diol in place of ethylene glycol.

The copolyester of the heat-sealable layer is preferably a polymer whichis amorphous or substantially amorphous, i.e. non-crystalline. In apreferred embodiment, the amorphous polymer of the heat-sealable layerhas a glass transition temperature (Tg) of at least 40° C., preferablyat least 70° C. and more preferably at least 80° C.

The thickness of the heat-seal layer is generally between about 1 and30% of the thickness of the substrate layer. The heat-sealable layer mayhave a thickness of up to about 50 μm, preferably up to about 25 μm,more preferably up to about 15 μm, more preferably up to about 10 μm,more preferably between about 0.5 and 6 μm, and more preferably betweenabout 0.5 and 4 μm. A thicker heat-sealable layer will generally form astronger heat-seal bond, which may result in tearing of the film inopening.

Formation of the heat-sealable layer on the substrate layer may beeffected by conventional techniques, for example by coating (generallyoff-line coating) or casting the polymer of the heat-sealable layer ontoa pre-formed substrate layer. Formation of the heat-sealable layer andthe substrate may also be effected by co-extrusion as is well-known inthe art and described herein. The coextruded sheet is stretched toeffect molecular orientation of the substrate, and preferably heat-set,as herein before described. For most polymers used in the heat-sealablelayer, the conditions applied for stretching the substrate layergenerally induce partial crystallisation of the heat-sealable polymerand it is therefore preferred to heat set under dimensional restraint ata temperature selected to develop the desired morphology of theheat-setting layer. In general, by effecting heat-setting at atemperature below the crystalline melting temperature of theheat-sealable polymer and permitting or causing the composite to cool,the heat-sealable polymer will remain essentially crystalline.Heat-setting at a temperature greater than the crystalline meltingtemperature of the heat-sealing polymer, will produce an essentiallyamorphous heat-sealable polymeric layer. Thus, heat-setting a compositesheet comprising a polyester substrate and a copolyester heat-sealablelayer at a temperature within a range of from 175 to 200° C. generallyyields a substantially crystalline heat-sealable layer. Using atemperature of from 200 to 250° C. generally yields an essentiallyamorphous heat-sealable layer. Certain polymers suitable for use in aheat-sealable layer, such as the PETO referenced above, remain amorphousat all times.

In one embodiment of the invention, the composite structure comprisingthe substrate layer and the heat-sealable layer may be provided bybiaxially-oriented polyester films having a heat-sealable layer whichare commercially available as MYLAR® OL film or MYLAR® WOL film(including grades OL, OL2, OL12, OL13, WOL and WOL12; DuPont TeijinFilms). Mylar OL is a dual ovenable film which provides peelable sealsto polar materials such as amorphous polyester (APET, also PETG),semicrystalline polyester (CPET), polyester coated paperboard andpolyvinylchloride (PVC). The heat-seal bond to the container isgenerally effected in the range from about 140 to about 200° C.

In an alternative embodiment of the invention, the composite structurecomprising the substrate layer and the heat-sealable layer may beprovided by biaxially-oriented polyester films having an EVAheat-sealable layer which are commercially available as MYLAR® RL film(including grades RL42, RL43; DuPont Teijin Films). Mylar RL is amicrowaveable film which provides peelable seals to polypropylene,poystryrene and polyethylene and also to polar materials such as APET,CPET, PVC and PVdC. The heat-seal bond to the container is generallyeffected in the range from about 120 to about 160° C.

The shrinkable layer is a self-supporting film or sheet by which ismeant a film or sheet capable of independent existence in the absence ofa supporting base. The shrinkable layer comprises any suitable polymericmaterial provided that it satisfies the requirements of a degree ofshrinkage in a first dimension of about 10 to 80%, and preferably about50 to 80%, over the temperature range 55 to 100° C., and a ratio ofshrinkage at 100° C. in said first dimension relative to a second,orthogonal dimension in the range from about 1:1 to 10:1, preferablyabout 2:1 to 8:1, and more preferably from about 3:1 to 7:1. Typically,the shrinkage in one dimension is about 4:1, for instance one dimensionof the film shrinks by about 75% of its size while the orthogonaldimension remains unchanged.

The shrinkable layer preferably comprises thermoplastic polymericmaterial. Such materials include a homopolymer or copolymer of a1-olefin, such as ethylene, propylene and but-1-ene, polyamides,polycarbonates, polyesters (including copolyesters), PVC, PVA,polystyrenes, polyacrylates, celluloses and nylon (including nylon 6 andnylon 6,6). Particularly preferred is a polyester material, andparticularly a synthetic linear polyester.

The synthetic linear polyesters useful for formation of the shrinkablelayer may be obtained by condensing one or more dicarboxylic acids ortheir lower alkyl diesters, e.g. terephthalic acid, isophthalic acid,phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinicacid, sebacic acid, adipic acid, azeleic acid, 4,4′-diphenyldicarboxylicacid, hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly an aliphatic or cycloaliphatic glycol,e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycoland 1,4-cyclohexanedimethanol. Aromatic dicarboxylic acids arepreferred. Aliphatic glycols are preferred.

In one embodiment, the polyester of the shrinkable layer is selectedfrom polyethylene terephthalate (PET) and polyethylene naphthalate(PEN), and preferably from polyethylene terephthalate.

In a preferred embodiment, the shrinkable layer comprises a copolyesterof terephthalic acid (TPA) and isophthalic acid (IPA) with one or morediols selected from the group consisting of aliphatic and cycloaliphaticdiols. Preferably, the molar ratios of the isophthalate polyester unitsto the terephthalate polyester units are from 1 to 40 mol % isophthalateand from 99 to 60 mol % terephthalate, preferably from 15 to 20 mol %isophthalate and from 85 to 80 mol % terephthalate. The terephthalicacid/isophthalic acid copolyester may be obtained by condensationpolymerisation of terephthalic acid and isophthalic acid with one ormore, and preferably one, diol as set out above, preferably an aliphaticdiol, more preferably ethylene glycol. In a particularly preferredembodiment, the shrinkable layer comprises a copolyester comprisingsubstantially 18 mol % ethylene isophthalate and 82 mol % ethyleneterephthalate.

In an alternative embodiment, the shrinkable layer comprises a copolymerof terephthalic acid, dihydroxyethyl azeleic acid and ethylene glycol,preferably wherein the dicarboxylic acid components of the copolymer arein a molar ratio of about 90-99% terephthalic acid and about 1-10%dihydroxyethyl azeleic acid, particularly about 95% terephthalic acidand about 5% dihydroxyethyl azeleic acid.

The shrinkable layer may itself comprise a monolayer or a multilayerstructure, particularly an ABA type structure, particularly where thecore layer is a layer comprising a polymer as described above in respectof the preferred embodiment. In such an ABA-type structure, the A layerspreferably comprise a polyester or copolyester as described herein,particularly a copolyester of terephthalic acid with about 30-35 mole %,preferably about 33 mole % 1,4-cyclohexane dimethanol and about 65-70mole %, preferably about 67 mole % ethylene glycol.

The thickness of the shrinkable layer is generally in the range of about10 to about 100 μm, preferably about 25 μm to about 75 μm, and typicallyabout 50 μm.

The shrinkable layer may be produced in accordance with the conventionalfilm-forming procedures already described herein. In the preferredembodiment described above, the shrinkable layer is stretched only inone direction and not heat-set. Conveniently, the film is stretched onlyin the transverse direction. As noted above, stretching is effected toan extent determined by the nature of the polymer. For example, PET isusually stretched so that the dimension of the oriented film in thedirection of stretching is from about 2 to 8 times, preferably fromabout 2 to about 5 times, and more preferably between about 3 and 4times its original dimension.

The intermediate layer acts as a “heat-sink” to prevent the filmoverheating, which may lead to melting or delamination of thecopolyester layers, and should have a relatively low dielectric constantand a relatively high combustion point. In one embodiment theintermediate layer has a dielectric constant of less than about 10,preferably less than about 6, more preferably in the range from about0.2 to about 6, more preferably in the range from about 0.2 to about 4(measured, for instance, at 50 Hz). In a further embodiment, theintermediate layer has a melting or combustion point of more than about180° C., preferably more than 200° C., preferably more than about 220°C., and preferably more than about 240° C. It may be made of anysuitable film-forming material but is conveniently made of paper orcellophane (or other cellulose-based film forming material, preferablyregenerated cellulose).

In the embodiments wherein the multilayer film is required to betransparent or translucent, the intermediate layer is suitablycellophane. In the embodiments where the multilayer film is required tobe opaque, the intermediate layer is suitably paper. The intermediatelayer may comprise a printable surface and be capable of displayingprinted information or graphics.

The thickness of the intermediate layer is generally in the range ofabout 12 to about 100 μm, preferably from about 20 to about 50 μm, andtypically about 25 μm.

The metallic layer is suitably produced by conventional metallisationtechniques well-known in the art. Deposition of a metallic layer may beeffected by conventional metallising techniques, for example, bydeposition from a suspension of finely-divided metallic particles in asuitable liquid vehicle, or by electron beam evaporation, electrolessplating, or preferably, by a vacuum deposition process in which a metalis evaporated onto the surface to be coated in a chamber maintainedunder conditions of high vacuum. The metallic layer may also be appliedby printing techniques, for example as disclosed in EP-A-0276654, thedisclosure of which is incorporated herein by reference. Suitable metalsinclude palladium, titanium, chromium, nickel, copper (and alloysthereof, such as bronze), silver, gold, cobalt and zinc, but aluminiumis to be preferred for reasons both of economy and ease of bonding.Metallising may be effected over the entire exposed surface of the layerto be coated or over only selected portions thereof, as desired.

The metallic layer is conveniently deposited in a thickness frommonoatomic proportions to about 50 μm or greater, although a preferredrange is from 0.005 to 15.0 μm and particularly from 0.01 to 0.5 μm.

The metallic layer may be produced by deposition of the metal by themetallisation techniques noted above on a surface of the substrate (oneor more of the surface(s) juxtaposed with the heat-sealable layer,shrinkable layer or the intermediate layer), or a surface of theintermediate layer (one or more of the surface(s) juxtaposed with thesubstrate, the heat-sealable layer, or the shrinkable layer), or asurface of the shrinkable layer (suitably the surface juxtaposed with anintermediate layer) or a surface of the heat-sealable layer (suitablythe surface juxtaposed with the substrate layer). Preferably, themetallic layer is deposited on either or both of the surface of thesubstrate layer juxtaposed with the intermediate layer or the surface ofthe shrinkable layer juxtaposed with the intermediate layer, and ispreferably deposited on the surface of the substrate layer juxtaposedwith the intermediate layer.

If desired, a primer layer may be applied, prior to metallisation, tothe surface of the layer to be metallised in order to increase theadhesion of the metal thereto. Such primer layers are disclosed in, forexample, EP-A-0348062 and EP-A-0375215, the disclosures of which areincorporated by reference.

One or more of the layers of the polymeric film may conveniently containany of the additives conventionally employed in the manufacture ofpolymeric films. Thus, agents such as cross-linking agents, dyes,pigments, voiding agents, lubricants, anti-oxidants, radical scavengers,flame retardants, UV absorbers, thermal stabilisers, anti-blockingagents, surface active agents, slip aids, optical brighteners, glossimprovers, prodegradents, viscosity modifiers and dispersion stabilisersmay be incorporated as appropriate. In particular, a layer may comprisea particulate filler which can improve handling and windability duringmanufacture. The particulate filler may, for example, be a particulateinorganic filler or an incompatible resin filler or a mixture of two ormore such fillers.

By an “incompatible resin” is meant a resin which either does not melt,or which is substantially immiscible with the polymer, at the highesttemperature encountered during extrusion and fabrication of the film.The presence of an incompatible resin usually results in a voided layer,by which is meant that the layer comprises a cellular structurecontaining at least a proportion of discrete, closed cells. Suitableincompatible resins include polyamides and olefin polymers, particularlya homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbonatoms in its molecule. Preferred materials include a low or high densityolefin homopolymer, particularly polyethylene, polypropylene orpoly-4-methylpentene-1, an olefin copolymer, particularly anethylene-propylene copolymer, or a mixture of two or more thereof.Random, block or graft copolymers may be employed.

Particulate inorganic fillers include conventional inorganic fillers,and particularly metal or metalloid oxides, such as alumina, silica(especially precipitated or diatomaceous silica and silica gels) andtitania, calcined china clay and alkaline metal salts, such as thecarbonates and sulphates of calcium and barium. The particulateinorganic fillers may be of the voiding or non-voiding type. Suitableparticulate inorganic fillers may be homogeneous and consist essentiallyof a single filler material or compound, such as titanium dioxide orbarium sulphate alone. Alternatively, at least a proportion of thefiller may be heterogeneous, the primary filler material beingassociated with an additional modifying component. For example, theprimary filler particle may be treated with a surface modifier, such asa pigment, soap, surfactant coupling agent or other modifier to promoteor alter the degree to which the filler is compatible with the primarypolymeric material of the layer.

Preferred particulate inorganic fillers include titanium dioxide andsilica.

Titanium dioxide particles may be of anatase or rutile crystal form. Thetitanium dioxide particles preferably comprise a major portion ofrutile, more preferably at least 60% by weight, particularly at least80%, and especially approximately 100% by weight of rutile. Theparticles can be prepared by standard procedures, such as the chlorideprocess or the sulphate process. The titanium dioxide particles may becoated, preferably with inorganic oxides such as aluminium, silicon,zinc, magnesium or mixtures thereof. Preferably the coating additionallycomprises organic compound(s), such as fatty acids and preferablyalkanols, suitably having from 8 to 30, preferably from 12 to 24 carbonatoms. Polydiorganosiloxanes or polyorganohydrogensiloxanes, such aspolydimethylsiloxane or polymethylhydrogensiloxane are suitable organiccompounds. The coating is suitably applied to the titanium dioxideparticles in aqueous suspension. The inorganic oxides are precipitatedin aqueous suspension from water-soluble compounds such as sodiumaluminate, aluminium sulphate, aluminium hydroxide, aluminium nitrate,silicic acid or sodium silicate. The coating layer on the titaniumdioxide particles is preferably in the range from 1 to 12% of inorganicoxides, and preferably in the range from 0.5 to 3% of organic compound,by weight based upon the weight of titanium dioxide.

The inorganic filler should be finely-divided, and the volumedistributed median particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value) thereof ispreferably in the range from 0.01 to 5 μm, more preferably 0.05 to 1.5μm, and particularly 0.15 to 1.2 μm.

The size distribution of the inorganic filler particles is also animportant parameter, for example the presence of excessively largeparticles can result in the film exhibiting unsightly ‘speckle’, i.e.where the presence of individual filler particles in the film can bediscerned with the naked eye. It is preferred that none of the inorganicfiller particles incorporated into the substrate layer should have anactual particle size exceeding 30 μm. Particles exceeding such a sizemay be removed by sieving processes which are known in the art. However,sieving operations are not always totally successful in eliminating allparticles greater than a chosen size. In practice, therefore, the sizeof 99.9% by number of the inorganic filler particles should not exceed30 μm, preferably should not exceed 20 μm, and more preferably shouldnot exceed 15 μm. Preferably at least 9%, more preferably at least 95%by volume of the inorganic filler particles are within the range of thevolume distributed median particle diameter ±0.8 μm, and particularly±0.5 μm.

Particle size of the filler particles may be measured by electronmicroscope, coulter counter, sedimentation analysis and static ordynamic light scattering. Techniques based on laser light diffractionare preferred. The median particle size may be determined by plotting acumulative distribution curve representing the percentage of particlevolume below chosen particle sizes and measuring the 50 th percentile.

The components of the composition of a layer may be mixed together in aconventional manner. For example, by mixing with the monomeric reactantsfrom which the layer polymer is derived, or the components may be mixedwith the polymer by tumble or dry blending or by compounding in anextruder, followed by cooling and, usually, comminution into granules orchips. Masterbatching technology may also be employed.

In one embodiment, the film of the present invention is optically clear,preferably having a % of scattered visible light (haze) of <10%,preferably <6%, more preferably <3.5% and particularly <2%, measuredaccording to the standard ASTM D 1003. In this embodiment, filler istypically present in only small amounts, generally not exceeding 0.5%and preferably less than 0.2% by weight of a given layer.

In an alternative embodiment, the film is opaque, preferably exhibitinga Transmission Optical Density (TOD) (Sakura Densitometer; type PDA 65;transmission mode) in the range from 0.1 to 2.0, more preferably 0.2 to1.5, more preferably from 0.25 to 1.25, more preferably from 0.35 to0.75 and particularly 0.45 to 0.65. For films which do not contain anopaque intermediate layer, a multilayer film is conveniently renderedopaque by incorporation into the polymer blend of an effective amount ofan opacifying agent. Suitable opacifying agents include an incompatibleresin filler, a particulate inorganic filler or a mixture of two or moresuch fillers, as hereinbefore described. The amount of filler present ina given layer is preferably in the range from 1% to 30%, more preferably3% to 20%, particularly 4% to 15%, and especially 5% to 10% by weight,based on the weight of the layer polymer.

The surface of an opaque film preferably exhibits a whiteness index,measured as herein described, in the range from 60 to 120, morepreferably 80 to 110, particularly 90 to 105, and especially 95 to 100units.

The surface of the substrate nearest the heat sealable layer is referredto herein as the primary side. The side of the substrate opposite to theside nearest the heat-sealable layer is referred to herein as thesecondary side. The secondary side of the substrate may have thereon oneor more further polymeric layers or coating materials. Any coating ofthe secondary side is preferably performed “in-line”.

In one embodiment, an additional coating on the secondary side maycomprise a “slip coating” in order to improve the handling andwindability of the film, particularly when the film substrate is a PETpolyester substrate. A suitable slip coating may be, for instance adiscontinuous layer of an acrylic and/or methacrylic polymeric resinoptionally further comprise a cross-linking agent, such as described inEP-A-0408197, the disclosure of which is incorporated herein byreference. An alternative slip coating may comprise a potassium silicatecoating, for instance as disclosed in U.S. Pat. Nos. 5,925,428 and5,882,798, the disclosures of which are incorporated herein byreference.

In a further embodiment, the film has on one surface thereof a printableor ink-receiving layer, and optionally a primer layer (such as thatdisclosed in EP-0680409, EP-0429179, EP-0408197, EP-0576179 orWO-97/37849, the disclosures of which are incorporated herein byreference) between the film and the printable or ink-receiving layer inorder to increase adhesion. Suitable printable or ink-receiving layersare disclosed in, for instance, EP-0696516, U.S. Pat. No. 5,888,635,U.S. Pat. No. 5,663,030, EP-0289162, EP-0349141, EP-0111819 andEP-0680409, the disclosures of which are incorporated herein byreference. The multilayer film may carry printed information at a numberof other locations, for instance on the intermediate layer as mentionedabove. It is also possible to “reverse print” the heat-sealable peelablelayer as is well known in the art.

In a further embodiment, the film has on one surface thereof a gasbarrier layer. In one embodiment, the gas barrier layer is applied tothe secondary surface of the substrate layer of the film usingconventional techniques. Gas barrier layers are well-known in the artand include, for instance, aluminium oxide, silicon oxides,polyvinylidene chloride (PVDC), polyvinyl alcohol, saponifiedethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer andvermiculite. Aluminum oxide layers can be applied using vacuumdeposition techniques, as known in the art and described herein.Polymeric gas barrier layers can be applied using conventional coatingtechniques, for instance as describe herein. Gas barrier layers, andtheir methods of application, are disclosed for instance inEP-A-0962506, JP-A4/331246, JP-A-6/32924, U.S. Pat. No. 4,927,689, U.S.Pat. No. 6,364,987, JP-A-188624, JP-A-9/241999, EP-A-0498569 andEP-A-0518647. In a preferred embodiment, the gas barrier layer isaluminium oxide. Preferably, the gas barrier layer is sufficient toprovide the film with an oxygen transmission rate of 1-10 cm³/m²/day, orlower.

According to a further aspect of the present invention there is provideda process for the production of a multi-layer film which comprises thesteps of:

-   -   (i) forming a polymeric substrate;    -   (ii) providing a polymeric heat-sealable peelable layer;    -   (iii) forming a shrinkable layer of polymeric film having a        degree of shrinkage in a first dimension of about 10-80% over        the temperature range 55 to 100° C. and a ratio of shrinkage at        100° C. in said first dimension relative to a second, orthogonal        dimension in the range from 1:1 to 10:1;    -   (iv) forming an intermediate layer of film-forming material;    -   (v) forming one or more metallic layer(s);    -   (vi) disposing an intermediate layer between the substrate layer        and the shrinkable layer and/or between the substrate layer and        the heat-sealable layer, preferably between the substrate layer        and shrinkable layer; and    -   (vii) laminating the shrinkable layer, intermediate layer,        substrate layer and heat sealable layer to form a multilayer        structure,        wherein said formation of said one or more metallic layer(s) is        effected by metallisation, prior to lamination, of one or more        surface(s) of one or more of the substrate, intermediate layer,        shrinkable layer and/or heat-sealable layer as described herein.

In the preferred embodiment where the intermediate layer is disposedbetween the substrate and the shrinkable layer, the heat-sealable isprovided on a first surface of the substrate layer, preferably byco-extrusion with the polymer of the substrate.

In a preferred embodiment, step (vii) above is effected by laminating afirst surface of the intermediate layer to a surface of the shrinkablelayer and laminating a second surface of the intermediate layer to thesecond surface of the substrate layer, preferably to the second surfaceof a substrate layer having on its first surface a co-extrudedheat-sealable layer.

The lamination process to form the multilayer structure may be effectedin respect of all layers simultaneously or alternatively may be effectedsequentially such that the multilayer structure is constructed with aseries of individual lamination steps between adjacent layers.Preferably lamination is effected simultaneously.

The lamination steps may use conventional laminating equipment andadhesives.

The adhesives suitable for use in preparing the films of this inventionare well-known to those skilled in the art, and include epoxy-,polyurethane- or acrylic-based one- or two-part systems. The adhesivemay be solvent-based (including water-based) or solvent-free. Theparticular adhesive chosen will depend on the compositions and the typesof layers that are present in the film. Generally, excellent bondstrengths can be obtained with high-performance two-component isocyanateadhesives such as Herbert's EPS 74/KN75. Solvent-free adhesives, such asHerbert's 1K-LF 190X3 or Herbert's 2K-LF 541/Hardener 110 may also beused. It is preferable to use solvent-free adhesives for theconstruction of the laminate as this reduces exposure of the shrinkablelayer to any unnecessary thermal cycles.

Prior to lamination of two layers, one or both of the opposingsurface(s) of the layers to be laminated may, if desired be subjected toa chemical or physical surface-modifying treatment to improve the bondbetween the two layers. Thus, prior to lamination of a pre-formedshrinkable layer onto a substrate, the exposed surface of the substratemay, if desired, be subjected to a chemical or physicalsurface-modifying treatment to improve the bond between that surface andthe subsequently applied pre-formed shrinkable layer. A preferredtreatment, because of its simplicity and effectiveness, which isparticularly suitable for the treatment of a polyolefin substrate, is tosubject the exposed surface of the substrate to a high voltageelectrical stress accompanied by corona discharge. Alternatively, alayer may be pretreated with an agent known in the art to have a solventor swelling action on the layer polymer. Examples of such agents, whichare particularly suitable for the treatment of a polyester layer,include a halogenated phenol dissolved in a common organic solvent e.g.a solution of p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or2,4,6-trichlorophenol or 4-chlororesorcinol in acetone or methanol.

The preferred treatment by corona discharge may be effected in air atatmospheric pressure with conventional equipment using a high frequency,high voltage generator, preferably having a power output of from 1 to 20kw at a potential of 1 to 100 kv. Discharge is conventionallyaccomplished by passing the film over a dielectric support roller at thedischarge station at a linear speed preferably of 1.0 to 500 m perminute. The discharge electrodes may be positioned 0.1 to 10.0 mm fromthe moving film surface.

As noted above, in one embodiment, a polymeric film according to theinvention is useful for sealing or providing a lid on a container, suchas a thermoformed tray, thermoformed bowl or blow-moulded bottle. Thecontainer may be formed of polyester, such as polyethyleneterephthalate, or of polypropylene, polystyrene, or may be PVDC coated,or may be glass, or maybe PET-coated cartonboard or paperboard. As notedabove, the multilayer film is particularly suitable for use on acontainer or tray which does not contain susceptor material or acontainer or tray which does not contain susceptor material in or on theseal area. A film according to the invention is particularly suitablefor use as a lid for an APET/CPET container, especially a thermoformedtray, which is suitable for packaging food or drink. The film is also,of course, capable of being used on other types of containers such as afoil container (such as an aluminium foil tray), or a metallised,susceptor container.

The invention further provides a sealed container comprising areceptacle containing food or drink, and a lid formed from a multilayerfilm as defined herein.

The multilayer film is used as a lidding film to seal a containeraccording to techniques well-known to those skilled in the art. Once thefood to be packaged has been introduced into the container, theheat-sealable film lid is affixed using temperature and optionallypressure using conventional techniques and equipment. The multilayerfilm lid is placed on the container such that the heat-sealable layer isin contact with the surfaces of the container and the shrinkable layeris the outermost surface of the film lid.

It is preferred that a portion of the film lid in contact with thecontainer forms a stronger heat-seal bond with the container than thatformed between the container and the remainder of the film lid incontact with the container. In this way, the film lid is more stronglyattached to the container during the cooking cycle and remains attachedto one side of the container during the self-peeling process, therebyensuring that the film lid peels back on itself during the cookingcycle. Thus, for a quadrilateral container, such as a rectangular orsquare container, it is preferred that the heat-seal bond on one side ofthe container is stronger that the heat-seal bond on the other threesides. For a circular or oval container, it is preferred that onesectional arc of the substantially circular or elliptical circumferenceof the open end of the container, for instance an arc comprising lessthan about 180°, preferably less than about 90°, preferably less thanabout 60°, and preferably less than about 45° of the circumference, hasa stronger heat-seal bond than the remaining section of thecircumference. Varying strengths of heat-seal bond may be achieved byforming different sections of the heat-seal bond between the film lidand the container at different temperatures. Thus, for a quadrilateraltray for instance, it is preferred that the heat-sealing equipment isconfigured such that three of the sides of the heat-seal head operate ata temperature in the range of about 110 to 150° C., preferably at about130° C., and the fourth side operates at a temperature in the range ofabout 150 to 200° C., preferably about 170° C.

In a particularly preferred embodiment, the film lid is folded over theflange on this fourth side of the container and the heat-seal head alsoforms a heat-seal bond between the film lid and the underside of theflange of the container on this fourth side. Ideally the film would befolded over the flange on the fourth side and sealed top and bottom.

In a further embodiment, the bond between the film lid and one side ofthe container may be enhanced by mechanical means, such as crimping orstapling, in order to ensure that the lid peels back on itself duringthe cooking cycle.

Also, as noted above, the film of the present invention is useful as thesole packaging means in an ovenable ready-prepared meal in which theseal is provided by heat-sealing a first portion of the film to a secondportion of the film. Such seals are effected by conventional techniquesand include “fin seals” and “overlap seals”, preferably fin seals.

Once the food product is placed within the film, the two portions of thefilm which are to be bonded together are brought together with the heatsealable surface of one film portion being in contact with the heatsealable surface of one film portion being in contact with the heatsealable surface of the other film portion, and the heat-seal bondformed by the application of temperature and optionally pressure usingconventional equipment. The heat-seal bond may be formed at temperaturesin the range of about 110 to about 150° C.

The invention further provides a packaged, sealed food product whereinthe packaging which effects and forms the seal around the food productis a multilayer film heat-sealed to itself, as defined herein.

The multilayer film is particularly suitable for use as packaging forready prepared convenience foods which are intended to be warmed in amicrowave oven. However, the invention is also applicable forready-prepared meals which are intended to be warmed in any other typeof oven, such as a conventional convection oven, a direct radiation ovenand a forced hot air oven.

In a further aspect of the invention there is provided use of theheat-sealable peelable multilayer laminated polymeric film describedherein as, or for the purpose of providing, packaging for ovenable mealswherein the packaging is self-peeling and self-venting during a cookingcycle.

In a further aspect of the invention there is provided use of theheat-sealable peelable multilayer laminated polymeric film describedherein as, or for the purpose of providing, a heat-sealable lid on anovenable container wherein said heat-sealable lid is self-peeling andself-venting during a cooking cycle.

The invention is illustrated by FIGS. 1 to 8 herein, in which:

FIGS. 1 and 2 are sectional views of embodiments of the film (1)according to the present invention which comprises a shrinkable layer(2), an intermediate layer (7), a metallic layer (8), a substrate layer(3) and a heat-sealable layer (4).

FIG. 3 is a sectional view of an alternative embodiment of the film (1)according to the present invention which comprises a shrinkable layer(2), a metallic layer (9), an intermediate layer (7), a metallic layer(8), a substrate layer (3) and a heat-sealable layer (4).

FIG. 4 is a sectional view of the film (1) after having been heat-sealedto the container (5) having food product (6) therein. The heat-seallayer is in contact with the container, with the shrinkable layeruppermost.

FIG. 5 is a view of the container (5) having walls (a), (b), (c) and(d). In a preferred embodiment of the invention, the film lid is heatsealed to the tray such that the heat-sealed lid has a stronger bond tothe upper surface of wall (a) than to sides (b), (c) and (d).

FIG. 6 is a sectional view of container (5) having walls (a), (b), (c)and (d) (walls (b) and (d) not shown) and having at the top of walls (a)and (c) flanges (6 a ) and (6 c ). According to a preferred embodimentof the invention, film (1) is heat-sealed to both the upper and lowersurfaces of flange (6 a ).

FIGS. 7 and 8 are sectional views of a film (1) containing food product(6) therein. The film is sealed to itself at fin seal (11) therebyenclosing the food product. The heat-sealable layer (4) and substratelayer (3) are shown. Layer (10) represents the metallic layer(s), theintermediate layer and the shrinkable layers. FIG. 7 shows a filmwherein the outer edges of the substrate are not covered with a metalliclayer, an intermediate layer or a shrinkable layer. However, themetallic layer, or the metallic layer and the intermediate layer, or themetallic layer and the intermediate layer and the shrinkable layer couldextend to the edge of the substrate layer. FIG. 8 shows a film whereinthe metallic layer and the intermediate layer and the shrinkable layerextend to the edge of the substrate layer.

The following test methods may be used to determine certain propertiesof the polymeric film:

-   -   (i) Wide angle haze is measured using a Hazegard System XL-211,        according to ASTM D 1003-61.    -   (ii) Whiteness index is measured using a Colorgard System 2000,        Model/45 (manufactured by Pacific Scientific) based on the        principles described in ASTM D313.    -   (iii) Heat-seal strength is measured as follows. The laminated        film is sealed, by means of the heat-sealable layer, to a        standard APET/CPET tray using a Microseal PA 201 (obtained from        Packaging Automation Ltd, England) tray sealer at a temperature        of 180° C., and pressure of 80 psi for two seconds. Strips of        the sealed film and tray are cut out at 90° to the seal, and the        load required to pull the seal apart measured using an Instron        operating at a crosshead speed of 0.2 mmin⁻¹. The procedure is        generally repeated 4 times, and a mean value of 5 results        calculated.    -   (iv) Shrinkage at a given temperature is measured by placing the        sample in a heated water bath at that temperature for 30        seconds. The shrinkage behaviour over the range 55 to 100° C. is        assessed using a number of film samples at different        temperatures over this range, generally at intervals of 5-10° C.

The invention is further illustrated by the following examples. It willbe appreciated that the examples are for illustrative purposes only andare not intended to limit the invention as described above. Modificationof detail may be made without departing from the scope of the invention.

EXAMPLES Example 1

Preparation of the Composite Film Comprising the Substrate and theHeat-Sealable Layer

A polymer composition comprising polyethylene terephthalate was extrudedand cast onto a cooled rotating drum and stretched in the direction ofextrusion to approximately 3 times its original dimensions. The film waspassed into a stenter oven at a temperature of 100° C. where the filmwas stretched in the sideways direction to approximately 3 times itsoriginal dimensions. The biaxially stretched film was heat-set at about230° C. by conventional means. The heat-set film was then coatedoff-line using conventional coating means with a copolyester of azeleicacid/terephthalic acid/ethylene glycol (45/55/100) to give a dry coatingthickness of 2 μm. The total film thickness was 20 μm.

Preparation of the Shrinkable Layer

A polymer composition comprising a copolyester of 18 mol % ethyleneisophthalate and 82 mol % ethylene terephthalate was melt-extruded andcast onto a cooled rotating drum. The film was passed into a stenteroven at a temperature of about 90° C. where the film was dried andstretched in the sideways direction to approximately 3.8 times itsoriginal dimensions. The total thickness of the final film was 50 μm.

The substrate layer was metallised with aluminium using a vapourdeposition technique onto the opposite side of the film than that coatedwith the azeleic acid/terephthalic acid/ethylene glycol copolymer. Thethickness of the metallic layer was approximately 20 μm.

Preparation of the Laminated Film According to the Present Invention

The multilayer laminated film of the present invention is prepared bylaminating together the shrinkable film and the substrate/heat-sealablecomposite film, prepared as described above, with an intermediate layercomprising cellophane (UCB Wigton, UK; dielectric constant 0.38;combustion point 242° C.), of thickness 25 μm. The intermediate layerwas sandwiched between the metallised surface of the substrate layer andthe shrinkable layer. The lamination is effected according toconventional techniques using a First Laminator machine (TaiseiLaminator Company Ltd.). The adhesive used in the lamination wasNovacote™ 2525/3 two-part adhesive.

Preparation of a Sealed Container with Lid According to the PresentInvention

The multilayer film prepared in accordance with the above procedure washeat-sealed to a PET-covered cartonboard tray (Trykko Pack A/S,Denmark), using a Sentinel Heat Sealer (Packaging Industries, USA) at atemperature of 130° C. and a pressure of 40 psi for 0.5 sec on threesides and a temperature of 160° C. and a pressure of 80 psi for 0.5seconds on the fourth side of the tray.

The sealed container was placed in a 900W microwave oven for 2 minutes,and removed from the oven at the end of the heating cycle. The film lidof the container had peeled back from three sides of the container andremained attached to the fourth side.

Preparation of a Sealed Product According to the Present Invention

A dough-based food product was placed on a multilayer film prepared inaccordance with the above procedure. The food product was wrapped in thefilm which was then heat-sealed to itself to form a fin-seal as shown inFIG. 8 using conventional equipment at a temperature of 130° C. for 0.5sec.

The sealed product was placed in a 900W microwave oven for 2 minutes,and removed from the oven at the end of the heating cycle. The film hadpeeled back from itself, from the fin seal, to reveal the food productlying exposed on the film.

Example 2

A laminated film was prepared in accordance with Example 1 except thatthe shrinkable layer was prepared as follows:

A polymer composition comprising a copolyester of 18 mol % ethyleneisophthalate and 82 mol % ethylene terephthalate was co-extruded with acopolyester of terephthalic acid/1,4-cyclohexane dimethanol/ethyleneglycol (100/33/67) to form an ABA structure, the core layer (B) beingthe ethylene isophthalate/ethylene terephthalate copolyester. The filmwas passed into a stenter oven at a temperature of about 90° C. wherethe film was stretched in the sideways direction to approximately 3.8times its original dimensions.

Example 3

A laminated film was prepared in accordance with Example 1 except thatthe intermediate layer was 45 gsm bleach craft paper (United PaperMills, Finland).

1. A heat-sealable peelable multi-layer laminated polymeric filmcomprising a polymeric substrate layer having on one side thereof apolymeric heat-sealable peelable layer and having on the opposite sidethereof a polymeric shrinkable layer, wherein said shrinkable layer hasa degree of shrinkage in a first dimension of about 10-80% over thetemperature range 55 to 100° C., and a ratio of shrinkage at 100° C.said first dimension relative to a second, orthogonal dimension in therange of 1:1 to 10:1, and further comprising one or two intermediatelayer(s) wherein an intermediate layer is disposed between the substratelayer and the shrinkable layer and/or between the substrate layer andthe heat-sealable peelable layer, and one or more layer(s) of anelectrically conductive material.
 2. A film according to claim 1 whereinsaid shrinkable layer has a degree of shrinkage in one or both of saidfirst and second dimension(s) that is greater than the degree ofshrinkage of the substrate layer in said dimension(s).
 3. A filmaccording to claim 1 or 2 wherein the shear force producible by theshrinkage of the shrinkable layer is greater than the strength of theheat-seal bond provided by the heat-sealable layer.
 4. A film accordingto any preceding claim wherein the substrate comprises polyester.
 5. Afilm according to any preceding claim wherein the substrate comprisespoly(ethylene terephthalate).
 6. A film according to any preceding claimwherein the shrinkable layer comprises a copolyester of terephthalicacid (TPA) and isophthalic acid (IPA) with one or more diols selectedfrom the group consisting of aliphatic and cycloaliphatic diols.
 7. Afilm according to claim 6 wherein said copolyester comprisesisophthalate polyester units and terephthalate polyester units in molarratios of from 1 to 40 mol % isophthalate and from 99 to 60 mol %terephthalate.
 8. A film according to claim 6 or 7 wherein thecopolyester comprises ethylene glycol.
 9. A film according to claim 6, 7or 8 wherein the shrinkable layer comprises a copolyester comprisingsubstantially 18 mol % ethylene isophthalate and 82 mol % ethyleneterephthalate.
 10. A film according to any preceding claim wherein theheat-sealable peelable layer comprises a copolyester of an aromaticdicarboxylic acid, an aliphatic dicarboxylic acid and a glycol.
 11. Afilm according to claim 10 wherein said copolyester of the heat-sealablepeelable layer comprises terephthalic acid, azeleic acid and ethyleneglycol.
 12. A film according to claim 11 wherein said copolyester is acopolyester of ethylene glycol with about 55% terephthalic acid andabout 45% azeleic acid.
 13. A film according to any of claims 1 to 9wherein the heat-sealable peelable layer comprises a copolyester derivedfrom an aliphatic diol, a cycloaliphatic diol and a dicarboxylic acid.14. A film according to claim 13 wherein the heat-sealable peelablelayer comprises a copolyester of terephthalic acid with ethylene glycoland 1,4-cyclohexanedimethanol.
 15. A film according to claim 14 whereinthe heat-sealable peelable layer comprises a copolyester of terephthalicacid with ethylene glycol and 1,4-cyclohexanedimethanol wherein themolar ratio of 1,4-cyclohexanedimethanol to ethylene glycol is in therange from 20:80 to 40:60.
 16. A film according to claim 15 wherein theheat-sealable peelable layer comprises a copolyester of terephthalicacid with about 33 mole % 1,4-cyclohexane dimethanol and about 67 mole %ethylene glycol.
 17. A film according to any of claims 1 to 16 whereinthere is one intermediate layer.
 18. A film according to any precedingclaim wherein the intermediate layer is between the substrate and theshrinkable layer.
 19. A film according to any of claims 1 to 16 whereina second intermediate layer is present such that there is a firstintermediate layer between the substrate and the shrinkable layer and asecond intermediate layer between the substrate and the heat-sealablepeelable layer.
 20. A film according to any preceding claim wherein saidintermediate layer has a dielectric constant of less than
 6. 21. A filmaccording to any preceding claim wherein the intermediate layercomprises paper or cellophane.
 22. A film according to any precedingclaim wherein said electrically conductive layer(s) is/are disposedbetween one or more of (i) the shrinkable layer and the intermediatelayer; and (ii) the intermediate layer and the substrate layer.
 23. Afilm according to any preceding claim wherein there is only oneelectrically conductive layer.
 24. A film according to any precedingclaim comprising an electrically conductive layer between the substrateand the intermediate layer.
 25. A film according to any preceding claimwherein the electrically conductive layer(s) is/are metallic layer(s).26. A sealed container comprising a receptacle containing food or drink,and a lid formed from a polymeric film according to any of claims 1 to25.
 27. A packaged sealed food product wherein the packaging whicheffects and forms the seal around the food product is a polymeric filmaccording to any of claims 1 to
 25. 28. A process for the production ofa heat-sealable peelable multi-layer film which comprises the steps of:(i) forming a polymeric substrate; (ii) providing a polymericheat-sealable peelable layer; (iii) forming a shrinkable layer ofpolymeric film having a degree of shrinkage in one dimension of about10-80% over the temperature range 55 to 100° C. and a ratio of shrinkageat 100° C. in said first dimension to a second, orthogonal dimension inthe range from 1:1 to 10:1; (iv) forming an intermediate layer offilm-forming material; (v) forming one or more metallic layer(s); (vi)disposing an intermediate layer between the substrate layer and theshrinkable layer and/or between the substrate layer and the heatsealable layer; and (vii) laminating the shrinkable layer, intermediatelayer, substrate layer and heat-sealable layer to form a multilayerstructure, wherein said formation of said one or more metallic layer(s)is effected by metallisation, prior to lamination, of one or moresurface(s) of one or more of the substrate, intermediate layer,shrinkable layer and/or heat-sealable layer.
 29. A process according toclaim 28 wherein the intermediate layer is disposed between thesubstrate layer and the shrinkable layer.
 30. A process according toclaim 29 wherein the heat-sealable layer is provided on a first surfaceof the substrate.
 31. A process according to claim 28 wherein thecomponents of the multilayer film are as set out in any of claims 1 to25.
 32. Use of a heat-sealable peelable multilayer laminated polymericfilm as set out in any of claims 1 to 25 as or for the purpose ofproviding packaging for ovenable meals wherein the packaging isself-peeling and self-venting during a cooking cycle.
 33. Use of aheat-sealable peelable multilayer laminated polymeric film as set out inany of claims 1 to 25 as or for the purpose of providing a heat-sealablelid on an ovenable container wherein said heat-sealable lid isself-peeling and self-venting during a cooking cycle.
 34. A useaccording to claim 32 wherein said polymeric film is the sole packagingfor the ovenable meal.
 35. A use according to claim 32, 33 or 34 whereinsaid ovenable container is suitable for use in a microwave oven.